Allium leaf-mining fly, techniques and treatments to avoid and cure the disease

Leaf miner symptoms on an allium leaf

Appearing in eastern France in 2003, allium leaf-mining flies have continuously spread West and are today among the most devastating enemies of leek overall.

If any parasite is tunneling through your growing leek, it is sure to be the allium leaf-mining fly! Even though they are quite hard to eradicate, much can be done to avoid their appearance…

A small, gray fly 1/10 inch (3 mm) long with latin name Phytomyza gymnostoma, leaf-mining flies attack leek twice a year, in spring and in fall.

Although leek is their primary target, leaf-miner pinworms can also attack chives, shallot and onion.

Conditions that favor the spread of allium leaf-mining flies

To this day, no study has formally determined which environmental conditions most favor the spread of allium leaf-mining flies.

Many factors are involved, like temperature, moisture levels or type of soil, but none of those stand out as being really crucial or straightforward.

Allium leaf-mining fly onslaught season

What is already ascertained is that allium leaf-miners actually have 2 flight seasons, in spring and in fall.

That’s why most attacks occur in April, May and June and then again at the end of summer, from August until November.

Larvae are yellow-colored, and then mutate into brown-reddish pupae before becoming an adult fly.

Allium leaf-mining fly lifecycle

  • Male flies fertilize females. Females create wounds on leaves to create a food source and lay their eggs. These wounds are marked with little white dots on the top of the leaves.
  • Once these wounds are prepared on top of the leaves, the female fly lays its eggs inside the wound, where they hatch and become yellow-colored larvae.
  • The larvae dig through the leek, tunneling down to its base as they feed.
  • At the end of the tunnel, grown larvae then turn into brownish-red pupae.
  • The pupae mature inside the leek plant all summer long until they become adult flies. These new flies are the ones that fly at the end of summer and in fall to breed a second cycle.
  • Fall pupae overwinter inside the leek and start the cycle again in spring.
  • As you can see, if infested plants aren’t removed and destroyed, allium leaf-mining flies will cycle through the fertilization and laying process over and over for years.

Fighting and treating allium leaf-mining fly infestations

Today, there are no curative treatments against allium leaf-mining flies.

However, it is possible to keep them from attacking your leek plants by providing adequate protection.

A net against leaf-mining flies

  1. First and foremost, the most effective protection is an anti-insect net, with a 1/5 inch (5 mm) mesh at most, preferably 850 µm or microns.
  2. Position hoops so that the net does not touch any part of the leek plant.
  3. Check that there is no single space more than 1/5 inch (5 mm) wide all around the net, even near the ground.
  4. Once risks of leaf-mining fly attacks are averted, the net can be removed.

Destroy infested leek plants

  1. Preventive measures also include getting rid of all infested plants.
  2. Do not compost them because the pupae could move out and into the compost and render it unusable.
  3. Make sure you have pulled out every single infection, because a single remaining larvae can lead to a new infestation.

Insecticides and repellent plants against allium leaf-mining flies

As of today, there are no effective insecticides against allium leaf-mining flies, neither conventional nor organic.

But it seems that some plants have beneficial properties, in that they repel leaf-mining flies.

  1. The best is to plant the leek in small bunches amidst these repellent plants.
  2. Such plants are mint, lemon balm or common rue.

Image credits (edits Gaspard Lorthiois):
CC BY 4.0: Elizabeth Axley